Re: 8" SCT focal ratio changes and magnification

On Nov 11, 6:57 pm, Tenifer <tensorsur...@xxxxxxxxx> wrote:

In a refractor. It is obviously from the objective lens to the rear
part where an object at infinity comes into focus. But in an
SCT. The converging of the rays doesn't start in the corrector!
The converging starts in the primary mirror. Hence, I said
that the focal length is the distance from the primary
mirror to the focal point.

The focal length of an SCT is the length one would have to extend the
cone of light outward from a star's image at the focal plane until
that cone of light is equal in diameter to the telescope's entrance
pupil. For a typical 8-inch f/10 SCT that distance will be
approximately 80 inches (6.7 feet). You won't come very close to that
figure by (incorrectly) measuring the distance from the primary mirror
to the focal point.

Bill Greer
To sketch is to see.

.

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